Human temporal bones obtained at autopsy from patients with histories of sensorineural hearing loss due to ototoxic drugs, noise exposure, aging, vascular disorders, and hereditary deafness will be studied by microdissection and phase-contrast examination of surface preparations of inner ear tissues. Pathological findings in the form of cytocochleograms will be correlated with audiometric data. Similar comparisons will be carried out in macaques and other genera of monkeys trained for behavioral audiometry and treated with aminoglycosides or exposed to various noise spectra. Similarly, ototoxic drugs, noise exposure, and combinations of these cochlear insults will be used in guinea pigs trained for behavioral audiometry and/or equipped with indwelling round window electrodes for chronic recording and analysis of the compound action potentials of the cochlear nerve. Pathological changes of the inner ears of experimental animals will be examined by light and electron microscopy, including the cochlear and vestibular neurons. In selected animals, cochlear and vestibular nuclei will also be examined for possible degenerative changes. Aged monkeys and dogs will be used as models for presbyacusis, and Dalmatian dogs as models for hereditary cochleo-saccular degeneration. A study of the effects of cochlear prostheses on inner ear structures will be made in the temporal bones of patients and monkeys that have received such implants, as they become available at autopsy.